There's a secret hiding underneath every concrete casting, beautiful aggregate. To make your concrete project really stand out this lesson covers how to expose the aggregate in your casting and give it a mirror finish.

Introduction: Concrete Finishing

Concrete Finishing is the process of working the surface of the concrete to achieve a desired effect. Since this class is focusing on craft and decorative ways to make concrete I'll only be covering the most popular finishing method here; exposed aggregate with a smooth finish.

Getting a smooth finish on concrete is as easy as having a smooth form to cast against, however you're left with a uniform grey finish to the concrete. By exposing the aggregate you get to see all the small details of each pebble and stone that's locked inside the concrete and you get to have a smooth finish, too!

In this lesson I'll share how to get that exposed aggregate look inexpensively and most importantly, safely.

Tools

In simplest terms, you need a grinder and PPE. But, it's a little more complicated than that (as always).

Here's where the power tools listed in Tools + Materials come into play. When you grind anything you're going to make dust, but when you grind concrete you create a very fine particulate that is hazardous to inhale. The safest way to abate this is to introduce water which binds to the dust and prevents it from becoming airborne. You can buy a special wet grinder which has a water drip function built right in, but it can be costly for someone just getting into casting concrete.

A fine alternative (and less messy) method to watering down the dust created by grinding is to try and remove it from the area quickly. This means you can use an inexpensive variable speed angle grinder and a large fan to quickly blow the dust out of the area.

It's important to know that with either method you're going to need the proper PPE.

When grinding concrete always use sealed eye protection and a cartridge dust mask that is NIOSH (National Institute of Occupational Safety and Health) approved. Both eye protection and dust mask should fit snugly to your face.

Since concrete is very hard we'll need an even harder material to grind and polish. Like sandpaper, diamond polishing pads come in different levels coarseness, from rough to smooth. This kit has doubles of the coarse grades of polishing discs, then steps up to very fine grade for a mirror finish.

These pads are interchangeable, so moving up a grade is really simple. Just like sanding wood, start with the coarsest grit and work your way up to the finer grits.

Changing Grinder Attachments

A variable speed grinder is important when grinding and polishing concrete as you want to have the control to slow down the rotation of the grinder to get a better feel of the concrete, rather than having the grinder spin at maximum speed which is which all regular angle grinders do.

To fit the concrete heads to the angle grinder the grinding wheel will need to be removed and replaced with the concrete head.

All angle grinders are a little different, but most will have a special nut that holds on the grinding wheel. This grinding wheel is meant to be interchangeable and sits on the rotating threaded shaft.

The special nut will have a corresponding tool that allows the nut to be removed. To take the nut off you'll need to engage the wheel brake on the grinder, typically a button on the top of the grinder. Press the brake and rotate the grinding wheel by hand until you feel the brake hold the grinding wheel in place, then attach the nut removal tool on the nut and remove the nut. The grinding wheel can then be pulled from the threaded shaft.

The concrete head will have a matching threaded insert that is just screwed onto the threaded shaft. Using the brake as before, thread the concrete head onto the grinder until it is fully seated.

You want the concrete head to sit parallel to the body of the grinder, and it should not interfere with the grinding guard at all.

The concrete grinder pads are interchangeable, so you can start at a coarse grit and work up the grits to very fine.

The pads for this grinder head are hook and loop (Velcro) and can just be stuck on. I found that while this style is very handy the pads can sometimes slip off center, when attaching them I usually give them a little twist to ensure they were fully seated on the grinder head.

Check to make sure the pads sit flat on the grinder head, this will ensure a flat and uniform grinding surface.

With the angle grinder set up with the polishing disks you're ready to get started.

When grinding a very hard surface like concrete you want a slow rotating grinding tool. Unlike cutting tools which are typically fast rotating and generate heat and debris, for grinding and polishing a slower speed is desired.

With a variable speed angle grinder the speed of the rotating head can be changed from slow to fast. The speed of the head will vary depending on the load, how much pressure is being applied to the head while grinding. Start on the lowest setting and apply moderate pressure. For the grinder I used I had to use slightly more than the lowest setting so the tool wouldn't stall and stop rotating.

Wearing full PPE (sealed eye protection, cartridge respirator, face shield), and having your large fan blowing any dust and debris away from you and your work area (or using a wet grinding method), start grinding at one edge.

Ensure that you heep the grinding face flat on the work surface and gently move the grinder in small circular motions, working your way along the surface.

Grinding and polishing is not quick work. Taking the time to start with coarse grinding disks (lower numbers) and working up through to the fine disks (higher numbers) you can achieve a consistent and beautiful finish. This is very similar to how sanding works in woodworking.

Take periodic breaks and inspect your work. The pressure applied to the tool will affect the surface finish, so consistent and light pressure is easier to maintain over the duration of grinding.

Finishing concrete exposes the aggregate and brings a new level of character to a piece. In the next lesson we'll explore a few finishes to apply as a topical treatment to take your concrete project to the next level.